Today, a primary concern of many climate scientists is the homogeneity
of the data sets they use, especially relative to some long-term baseline
such as 30-year decadal climate normals. Inhomogeneous data sets are the
result of changes in the biases associated with data measurement (discontinuities).

Homogenized data sets have been adjusted for discontinuities and provide
the means for data users to be confident that any changes and variations
identified in the data are natural and not artificial. The adjustments
are normally made so that the entire data set is based on the most recent
observing system. This concept is an extremely important one to the climate
community and requires a great deal of effort.

Inhomogeneous data sets can arise from a variety of factors, but in general,
can be classified into three basic groups related to:

Many of the noted countries sport expensive, high quality precipitation
gauges. These discontinuities also show up with climatological isoline
precipitation analysis at international boundaries.

The ratio of 10:1 snowfall-to-liquid precipitation has dropped from about
30-40 percent of all U.S. snowfall observations at the turn of the century,
to about 10 percent today (Figure 23). The frequency was derived for each
individual station and then averaged for several hundred long-term U.S.
stations.

Figure 23. Frequency of a Daily Ratio of 10:1 of Snowfall to Liquid
Equivalent Precipitation. (prepared by Ken Kunkel at the Midwestern
Regional Climate Center/Illinois State Water Survey).

An example of two types of other data continuity problems are shown in
Figures 24a and 24b.

Reno's busy urban airport has seen the growth of an urban heat bubble
on its north end. The corresponding graph of mean annual minimum temperature
(average of 365 night-time minimums each year) has as a consequence been
steadily rising. When the new ASOS sensor was installed, the site was
moved to the much cooler south end of the runway. Nearby records indicate
that the two cool post-ASOS years should have been warmer rather than
cooler. When air traffic controllers asked for a location not so close
to nearby trees (for better wind readings), the station was moved back.
The first move was documented, the second was not. The climate record
shows both the steady warming of the site, as well as the big difference
in overnight temperature between one end of this flat and seemingly homogeneous
setting, an observation borne out by automobile traverses around the airport
at night.

Another bias can creep into the climate record simply by changing the
sampling frequency or spatial averaging algorithms of the measuring and
processing systems. This bias can be removed readily as long as it is
quantified (by taking overlapping parallel observations between the old
and new systems) and documented in time in the station's history file
(metadata).

Perhaps the most difficult biases to compensate for occur when local
or micro-climatic changes around the instrumentation introduce non-representative
(but physically accurate spot scale) changes in the data record. These
inhomogenities can settle into the data quickly (a few days), as with
parking lot expansions, or gradually (months or longer), as in the case
of slowely maturing trees. The best known examples include the growth
of big-city urban heat islands, changes of land use, and stations relocations
(Figure 25). Any of these can blur or completely mask regional larger
scale climate variations and/or change.

Station relocations are unavoidable on occasion due to loss of an observer,
property ownership transfers, poor data quality, etc. When this situation
occurs, for whatever reason, the NWS field person is responsible for determining
whether the new site is "climatologically compatible" with the
old site and whether the data sets are thereafter treated as different
time series.

"compatibility is always determined by comparing the new to the
original equipment location for the station as described on Rendition
[original site, not last site] of the station's WS Form B-44. With some
exceptions, a move is considered compatible if the new equipment location
is within 5 miles of the original equipment location and the difference
in elevation is 100 feet or less. However, take great care to assure that
moves made within these limits are not, for example, from a hilltop to
a valley bottom or subject to other large magnitude influences such as
large water bodies, pavements, etc."

This somewhat arbitrary policy can result in the inappropriate continuation
of long-term data sets at climate stations which in fact may be incompatible.
Discontinuities such as these can then lead to serious misinterpretation
of climate variability and trends at the locale by data users.

Figures 26 a and 26 b illustrate an example of a relocation that resulted
in a large climate discontinuity although the station name remained the
same. This COOP station was moved from about 400 yards from this 100 foot-high
south-facing rock face to within about 100 feet of it. The original site
was an open sagebrush exposure. That summer, the station set or tied four
consecutive all time monthly extreme maxima.

This illustrates the importance of station exposure in determining climate
continuity. In this case, although the station's relocation fell well
under guidelines that require renaming (5 miles, 100 feet), the new setting
is completely incompatiable with the original site and needs to be started
over for climate purposes.

Figure 26a: Station Relocations are a Common Sources of Data Discontinuities
and Non-homogeneous Climate Records.

Figure 26 b: Relocated COOP Station With Same Name but Large Climate
Discontinuity.

NWSH Climate Services Division plans to revisit this policy in the near
future (FY05) to consider options for a more appropriate approach. However,
at this time, we discuss the situation within the limitations of the existing
policy.

Climate compatibility is best determined by running overlapping observations
at both the existing and new site for a minimum of a year or so and then
comparing differences. However, even when this approach is satisfied,
a quantitative definition of climate compatibility does not exist. Thus,
for the time being, it must be stated that climate continuity is subjectively
determined. Traditionally, this determination has been made on the spot,
on the day the station is moved, without benefit of analysis of actual
data behavior. Evidence is now suggesting that a number of those determinations
are in error and that many relocated stations should in fact be considered
as separate stations.

Recommended Station-Relocation Related
Actions

The following actions are recommended to minimize unwanted discontinuities
in the climate record:

1) When considering station relocation climatological compatibility,
strongly weight topographic characteristics of the old and new sites in
addition to the 5 mile/100 feet policy. In some cases the relevant tolerance
may be as low as 5-15 feet vertically and a few tens of feet horizontally.
Understand that topographic setting differences (i.e., slope orientation
and setting; crest, valley bottom, slope, plateau) can have a much greater
impact on data continuity than, say, the relocation coordinates in the
context of the 5-horizotal, 100-vertical feet rule. The chances are high
that the two stations are climatologically incompatible if there are any
significant topographic exposure differences.

2) Coordinate with your RCSPM and other climate services partners (NCDC,
RCCs, SCs) before making the final determination on station relocation
climate compatibility. Although current NWS policy gives you, the NWS
representative, responsibility for determining climatological compatibility
for station relocations, consultation and input from our climate partners
will assist you in making the best call possible.